With the progress in spread and miniaturization of mobile devices such as VTR, mobile phone and note PC, a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery has recently been used as a power supply therefor. Furthermore, in order to cope with recent environmental problems, the nonaqueous electrolyte secondary battery has also attracted interest as a power battery of an electric vehicle or the like.
Commonly, there has widely been used, as a positive electrode active material for a lithium ion secondary battery, LiCoO2 (lithium cobalt oxide) that can constitutes a 4 V-class secondary battery. When LiCoO2 is used as the positive electrode active material, it is put in practical use at a discharge capacity of about 160 mA/g.
Cobalt as a raw material of LiCoO2 is a scarce resource, which leads to high costs, and cobalt is unevenly distributed, which may cause anxiety about supply of a raw material.
In response to these circumstances, lithium transition metal complex oxide having a layered structure, such as lithium nickel cobalt manganese oxide obtained by substituting Co in LiCoO2 with one or more elements such as Ni and Mn has been developed. Generally, with respect to lithium transition metal complex oxide having a layered structure, its crystal structure becomes unstable when it has higher nickel ratio, and thus, it tends to cause precipitation of lithium compound in a positive electrode slurry upon manufacturing a positive electrode. In addition, when a cobalt ratio in the lithium transition metal complex oxide is decreased, the output power characteristics tend to be reduced.
By the way, there are techniques of selecting metal(s) such as tungsten as a substitution element depending on various purposes.
Patent document 1 discloses a technique of reducing a resistance of a positive electrode active material itself by adding one or more elements such as molybdenum and tungsten in lithium cobalt oxide and lithium nickel oxide.
Patent document 2 discloses lithium transition metal complex oxide in which up to about 20% of nickel in lithium nickel oxide is substituted with manganese, cobalt and the like, and up to about 10% of nickel is further substituted with tantalum, niobium, tungsten, molybdenum and the like. Patent document 2 describes that the lithium nickel oxide having a specific composition substituted with at least two elements in this manner exhibits improved thermal stability upon charging, and improved safety upon an internal short-circuit because of its lower electric conductivity in the form of powders.
Patent Document 3 discloses a technique of increasing an electrode plate density as well as enhancing thermal stability and load characteristics by lithium transition metal complex oxide having molybdenum, tungsten, boron and the like on its surface. As an average composition of the specific lithium transition metal complex oxide, lithium nickel cobalt manganese oxide comprising molybdenum is disclosed.
Patent document 4 discloses a technique of improving crystallinity of lithium transition metal complex oxide with preventing a sintering in a calcination stage by involving boron and the like (additive element 1) and molybdenum, tungsten, niobium, tantalum and the like (additive element 2) therein, which lead to satisfy all of a cost, high voltage resistance, high safety, rate characteristics and output power characteristics, and prevent a decrease in bulk density and an increase in specific surface area of powders. Specifically, lithium nickel cobalt manganese oxide comprising two aforementioned additive elements is disclosed.
On the other hand, there are techniques of mixing boron compound such as boric acid with lithium transition metal complex oxide, or techniques of lithium transition metal complex oxide having boron compound on its surface.
Patent document 5 discloses a technique of suppressing a reaction of lithium manganese oxide which has spinel structure with hydrohalic acid and improving cycling characteristics because of a positive electrode using lithium manganese oxide which comprises boron compound soluble in an electrolyte, such as boron oxide, orthoboric acid, metaboric id and tetraboric acid.
Patent document 6 discloses a technique of increasing a discharge potential and improving a lifetime characteristics by forming on a surface of lithium transition metal complex oxide a surface treatment layer which exhibits excellent ion conductivity, the surface treatment layer comprising hydroxide, oxyhydroxide and the like of a coating element such as boron. As a specific coating method, it discloses that the coating element dissolved in a solvent is precipitated on the surface of lithium transition metal complex oxide followed by removing the solvent.
Patent document 7 discloses a technique of preventing a gelation of an electrode paste by involving boric acid and the like as inorganic acid in an electrode which uses lithium transition metal complex oxide and the like. As a specific example of the lithium transition metal complex oxide, lithium nickel oxide is disclosed.
Patent document 8 discloses a technique of providing higher capacity of a secondary battery and improved charge-discharge efficiency of the secondary battery by attaching borate compound and the like such as ammonium borate and lithium borate on a surface of the particles of lithium transition metal complex oxide comprising nickel or cobalt indispensably, and by performing heat treatment under oxidizing atmosphere. As a specific example of the lithium transition metal complex oxide, lithium nickel oxide in which a portion of nickel is substituted with cobalt and aluminum is disclosed.
In any of the above-described Patent documents 5 to 8, a positive electrode and lithium transition metal complex oxide comprising tungsten is not disclosed.